1. Field of the Invention
The present invention relates to a lockup clutch for a torque converter, which is designed to connect a pump impeller and a turbine impeller of the torque converter directly to each other.
2. Description of the Related Art
It is conventionally and widely conducted in a torque converter that when the torque converter is brought into an operation state in which its amplifying function has almost been eliminated, a lockup clutch is automatically brought into a turned-on state to mechanically connect a pump impeller and a turbine impeller directly to each other, so that the loss by the sliding of the impellers relative to each other is eliminated. As a lockup clutch designed to be automatically turned on, a centrifugal weight-type lockup clutch is conventionally known, wherein a centrifugal weight is mounted to a driving plate connected to a pump impeller, so that when the rotational speed of the pump impeller is equal to or higher than a predetermined value, frictional engage portions of the lockup clutch are brought into engagement with one another by the centrifugal force of the centrifugal weight (for example, see Japanese Patent Application Laid-open No.60-18659).
The lockup clutch of the centrifugal weight type does not require a special control for operating the lockup clutch, but suffers from a disadvantage that its mechanical construction is complicated, resulting in an increase in cost.
Accordingly, it is an object of the present invention to provide a lockup clutch of the above-described type for a torque converter, which does not require a special control and moreover, is of a simple construction.
To achieve the above object, according a first aspect and feature of to the present invention, there is provided a lockup clutch for a torque converter, for connecting a pump impeller and a turbine impeller of the torque converter directly to each other, the lockup clutch comprising a pump extension connected to the pump impeller and surrounding the turbine impeller, a receiving plate connected to the pump extension to define, within the pump extension, a hydraulic pressure chamber communicating with an oil chamber defined between the pump impeller and the turbine impeller, a pressing plate opposed to the receiving plate for movement toward and away from the receiving plate, an annular friction clutch plate which is interposed between the receiving plate and the pressing plate and connected to the turbine impeller, a return spring for biasing the pressing plate in a direction away from the receiving plate, and an escape bore permitting the inside and outside of the receiving plate to communicate with each other on the side of an inner periphery of the friction clutch plate, wherein when the rotational speed of the pump impeller is increased to a value equal to or higher than a predetermined value, the pressing plate clamps the friction clutch plate. in cooperation with the receiving plate under the action of a centrifugal hydraulic pressure which is raised within the hydraulic pressure chamber in response to the increase in the rotational speed.
With the above arrangement, when the oil of the torque converter is filled in the hydraulic pressure chamber in the lockup clutch, and the rotational speed of the pump impeller is increased to the value equal to or higher than the predetermined value, the pressing piston clamps the friction clutch plate in cooperation with the receiving plate under the action of the centrifugal hydraulic pressure of the oil to connect the clutch cylinder and the pump impeller to each other. Therefore, it is possible to construct the lockup clutch into a type depending on the pump rotational speed, which is of a simple construction, as compared with the type using a centrifugal weight and does not require a special control means. Moreover, the oil used in this lockup clutch is the oil for operating the torque converter and hence, an exclusive oil pump is not required, and it is possible to provide the lockup clutch at an inexpensive cost.
According to a second aspect and feature of the present invention, there is provided a lockup clutch for a torque converter, for connecting a pump impeller and a turbine impeller of the torque converter directly to each other, the lockup clutch comprising a clutch cylinder connected to the turbine impeller, a pressing piston slidably received in a cylinder bore in the clutch cylinder to define a hydraulic pressure chamber, a piston-return spring for biasing the pressing piston toward the hydraulic pressure chamber, a means for introducing an oil flowing out of the torque converter into the hydraulic pressure chamber, and a frictional engage means provided between the clutch cylinder and the pump impeller, wherein when the rotational speed of the turbine impeller is increased to a value equal to or higher than a predetermined value, the pressing piston operates the frictional engage means under the action of a centrifugal hydraulic pressure which is raised within the hydraulic pressure chamber in response to the increase in the rotational speed to connect the clutch cylinder and the pump impeller to each other.
The means for introducing the oil into the hydraulic pressure chamber corresponds to an inlet bore 92 in an embodiment of the present invention which will be described hereinafter, and the frictional engage means corresponds to driving and driven friction clutch plates 85 and 86 in an embodiment of the present invention which will be described hereinafter.
With the above arrangement, when the oil flowing out of the torque converter is filled in the hydraulic pressure chamber in the lockup clutch, and the rotational speed of the turbine impeller is increased to the value equal to or higher than the predetermined value, the pressing piston operates the friction engage means under the action of the centrifugal hydraulic pressure of the oil to connect the clutch cylinder and the pump impeller to each other. Therefore, it is possible to provide a lockup clutch of the type depending on the turbine rotational speed which is of a simple construction, as compared with the type using a centrifugal weight, and a special control means is not required. Moreover, the oil used in this case is also the oil for operating the torque converter and hence, an exclusive oil pump is not required.
According to a third aspect and feature of the present invention, in addition to the second feature, the clutch cylinder is provided with an escape bore which permits an outer periphery of the hydraulic pressure chamber to be open to the outside, and a centrifugal valve which is operable to open the escape bore when the rotational speed of the clutch cylinder is lower than a predetermined value, and to close the escape bore when the rotational speed of the clutch cylinder is equal to or higher than the predetermined value.
With the above arrangement, when the rotational speed of the clutch cylinder is lower than the predetermined value, the remaining pressure in the hydraulic pressure chamber is released quickly through the escape bore by opening of the centrifugal valve, thereby enhancing the turning-off performance of the lockup clutch, and even foreign matters such as cut powder within the hydraulic pressure chamber can be discharged along with the oil. On the other hand, when the rotational speed of the clutch cylinder is equal to or higher than the predetermined value, the hydraulic pressure in the hydraulic pressure chamber can be raised by closing the centrifugal valve, and the operation of the lockup clutch cannot be impeded.
The above and other objects, features and advantages of the invention will become apparent from the following description of the preferred embodiments taken in conjunction with the accompanying drawings.